It is known that a single-axis (uniaxial) birefringence, which is characterized by two refraction indices, can be induced in a fiber-optic light waveguide (LWL) by a mechanical tension field which is transverse to the axis of the LWL. The propagation velocity of the light wave is accordingly dependent on the direction of the polarization. The extraordinary refraction index applies to the direction which is parallel to the tension axis (optical axis) and the ordinary refraction index applies to the direction which is vertical thereto. The differential of the refraction indices, which is often referred to as the relative birefringence, is a measurement for the applied mechanical tension.
Linearly polarized light is split vectorially in its oscillating direction into these two distinguished orthogonal directions and propagates at different velocities. After passing through the birefringent layer, the interference of the partial waves, particularly the two linear modes, can be effected by means of a polarizer. This also applies in principle for the polarized light of a white light source, e.g. a halogen lamp or a broad-band light-emitting diode (LED), but with the known restriction that interference (amplitude superposition) occurs only so long as the phase displacement does not substantially exceed the coherence length of the light wave. In this case, the intensities sum up to form a mean intensity which is not sensitive to the change in the birefringence.
Continuous phase displacement, as is possible in an interferometer, results in interference stripes which are amplitude-modulated with a so-called coherence function or white light interference.
Methods are discussed in the U.S. Pat. Application Ser. No. 07/795,725, which is assigned to the Assignee of the present application, the contents of which are hereby incorporated by reference herein, which enable a measurement of this phase difference. One of these methods is based on a compensating process. The magnitude of a control signal, which is required for the compensation, is then used as a measurement for the mechanical strain which acts on the sensor.
The degree of sensitivity to which the compensation can be adjusted is essential for a compensating process. This depends on the utilized light source, its coherence length and the autocorrelation.